Typical automatic transmissions include a torque converter and a powertrain of a multiple speed gear mechanism connected to the torque converter. In addition, a hydraulic control system is provided at an automatic transmission for selectively operating at least one operational element included in the powertrain according to a running state of a vehicle.
Such a hydraulic control system of an automatic transmission achieves an appropriate shift-speed corresponding to a vehicle speed and a throttle opening, by engaging/disengaging at least one operational element by supplying hydraulic pressure generated at an oil pump.
A solenoid valve is used for such engagement/disengagement of an operational element, and such a solenoid valve is usually realized as a duty control solenoid valve. A duty control solenoid valve is usually controlled by a pulse width modulation (PWM) signal.
As shown in FIG. 5, signals for operating the solenoid valve include a duty signal for a target shift-speed determined by a predetermined shift pattern based on a vehicle speed and a throttle opening, an over-excitation signal, and a chopping signal. The solenoid valve is driven by a driving signal obtained by synthesizing the above signals.
Applied current and motion of a plunger of the solenoid valve show a relationship as shown in FIG. 6, according to its electrical characteristics. When such a moving period in the motion of the plunger of the solenoid valve is plotted with respect to 0% to 100% of the applied duty, a characteristic graph shown as in FIG. 7 can be obtained.
As shown in FIG. 7, when an applied duty is small, the motion of the plunger shows a non-linear behavior with respect to the applied duty. In this case, the moving period of the plunger becomes longer than required, so hydraulic pressure supplied to a target frictional element becomes shorter than required.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.